Heme proteins are involved in a large number of physiologically important processes, including: delivery of oxygen to the tissues; derivation of stored energy from oxidation of nutrients; protective destruction of potentially dangerous peroxides and activation and elimination of toxic pollutants and carcinogens. Despite this remarkable functional diversity, all of these species employ the heme group as the active com- ponent. Identification of the factors responsible for this varied function would be of obvious benefit for attaining an understanding of the behavior in normal and abnormal states. The long term objective of this research program is the elucidation of the molecular mechanisms by which these various proteins interact with the common heme group to effectively regulate its reactivity. In order to clarify the molecular basis of functional behavior, a number of powerful spectroscopic techniques are employed to probe key interactions between the heme and the protein-and to determine the effect of subtle structural perturbations on the electronic structure and reactivity of the active sites and other key sites within-the protein. The principal spectroscopic methods to be employed during this phase of the work are resonance Raman and time-resolved resonance Raman (in both the visible and ultraviolet regions) which exploit novel rapid-mixing devices to yield structural information for short-lived intermediates. This approach applied to native and systematically modified proteins represents a realistic attempt to elucidate the important molecular control mechanisms responsible for such remarkable functional diversity.